Help to ID a lens?

Bit of an odd one this, but bear with me!

I recently bought a grubby Pentax MV and 28mm lens for the challenge of cleaning them up. I've done this and the camera works perfectly, but I can't identify the lens as the finishing ring is missing from the front along with the filter thread. I'd like to at least know who made it so I can look for a broken example to donate its front end.

All I know is:

It's a 28mm f2.8
It has full stops on the aperture ring with no half stop clicks
It was made in Japan
It's a Pentax K mount (not PKA)
It has macro markings
Infinity is at the right hand end of the focus scale.

Photos can be found here . Any help to ID this lens would be much appreciated!